1. Field of the Invention
The present invention relates to a proton conductor and a method of producing the same. In particular, the present invention relates to a proton conductor which exhibits an excellent proton conductivity and which hardly causes hydrolysis, and a method of producing the same.
2. Description of the Related Art
Recently, a proton conductor is used as an electrolyte for a fuel cell. In such a case, all components of the fuel cell can be constructed with solid materials, because the proton conductor is a solid. Therefore, it is possible to obtain a simple structure. Further, it is possible to decrease the frequency of maintenance operation, because no liquid leakage occurs.
The proton conductor, in which the proton conductivity is large, includes glasses obtained by compounding silicon oxide and Brønsted acid as described, for example, in Japanese Laid-Open Patent Publication Nos. 8-249923, 10-69817, and 11-203936. For example, the compound, which is obtained by doping silicon oxide with phosphoric acid as Brønsted acid, is adhered by a binder of any one of a thermoplastic elastomer (Japanese Laid-Open Patent Publication No. 8-249923), a polymer having sulfone group on the side chain (Japanese Laid-Open Patent Publication No. 10-69817), and a sulfonated product of block copolymer composed of conjugated diene unit and aromatic vinyl unit (Japanese Laid-Open Patent Publication No. 11-203936). Accordingly, the proton conductor is obtained.
The glass based on silicon oxide-phosphoric acid (hereinafter referred to as “Si—P-based glass”), which includes phosphoric acid in the SiO2 network structure as described in Japanese Laid-Open Patent Publication Nos. 8-249923, 10-69817, and 11-203936, the glass absorbs moisture or water in the atmospheric air, and it undergoes hydrolysis, resulting in liberation of orthophosphoric acid. C. Fernandez-Lorenzo et al. describes this phenomenon in Journal of Non-Crystalline Solids, Vol. 176, 1994, pp. 189–199 (especially, p. 197, left column, lines 24–28). Therefore, the proton conductivity of the Si—P-based glass may be lowered as the time elapses.
When the Si—P-based glass, which has the network-shaped SiO2 backbone doped with phosphoric acid (P2O5), is used as an electrolyte for a fuel cell, the proton conductivity of the electrolyte may be lowered as the time elapses. In such a situation, it is impossible to obtain predetermined power generation performance.